a. Field of the Invention
The present invention relates to a multimode optical fibre communications system, and in particular to an optical fibre communications system in which non-linearities in the propagation of the signal through a multimode optical communications channel degrade the signal presented to the receiver.
b. Related Art
An optical signal may be subject to numerous sources of noise and distortion, both in the generation of the signal and its transmission through an optical communications channel. Sources of noise in an optical transmitter or receiver include thermal noise and shot noise. A receiver section of the receiver may also use an avalanche photodiode for high sensitivity, but this will introduce avalanche photodiode noise. The generation of an optical signal will in general be subject to other sources of noise or drift. For example, the output power of a laser diode will be subject to slow drift as it heats up in use or from changes in ambient temperature. System distortion may be due to non-linear variations owing to the use of new or existing multimode optical fibre in an optical communications link. In a multimode optical fibre, different modes have different propagation velocities, which tends to disperse a pulse into adjacent pulses, thereby causing intersymbol interference (ISI). Pulse dispersion also occurs in single mode fibre, but to a lesser extent. Such effects will therefore tend to close an eye pattern and increase the measured bit error rate (BER) at the receiver.
Although changes owing to temperature drift can be compensated for quite easily, for example by temperature stabilisation of the laser source or with automatic gain control at the receiver, it is difficult to compensate for changes owing to pulse spreading in multimode optical fibres.
As a result, high-speed communications links, for example links operating at a data rate of at least 5 Gbit/s, have tended to use single mode optical fibre together with high precision optical fibre connectors. This has been the case even when such links are operating over short distances, such as in local area networks where links are typically of the order of 10 m to 100 m in length, or over medium distances, such as in metro networks where links are typically of the order of 1 km to 10 km in length. While such high-speed communications links provide reliable performance at a very low BER, for example 10−12, there is a need for comparable performance at greatly reduced cost, and in practice this requires the use of multimode optical fibre and cheaper connectors.
Therefore in recent years, multimode optical fibres have been used in communications links operating up to 10 Gbits/s over FDDI grade multimode fibre. With an 850 nm optical source, such links have been limited to about 30 m in length. With a 1310 nm optical source, such links have been limited to 80 m in length. It is, however, necessary in up to 90% of installed multimode optical fibre communications links to be able to transmit data over at least 300 m.
It has also been proposed to use an equaliser circuit at the receiver to compensate for intersymbol interference. Such an equaliser circuit receives as an input the output from a photodetector circuit and then generates from this at least two equaliser coefficients. A signal delay line also receives the output from the photodetector circuit. Tapped outputs from the delay line are each multiplied or otherwise combined with one of the equaliser coefficients, and then summed together to generate an equalised output signal. While this method can be effective in compensating for certain types of signal distortion, this adds additional cost to the receiver circuit and does not address the root cause of the problem of distortion due to pulse spreading in multimode optical fibre.
It is an object of the present invention to provide an optical communications system and a method of communicating an optical signal in such a system that addresses this problem.